Graphene At Play In New 300-Mile EV Battery

A research team from the Lawrence Berkeley National Laboratory has a 300-mile electric vehicle battery range in its sights, thanks to a unique combination of different electrochemical technologies including a new material called sulfur-graphene oxide (S-GO). CleanTechnica is one of the world’s leading fans of graphene so naturally we are most interested in (ok, so totally excited by) that. So, here goes.

Graphene To The Rescue

Sulfur has some key advantages over conventional lithium-ion battery technology in terms of storage capacity (far better), toxicity (none), cost (far less), and weight (ditto), but it is also very brittle.

Li-S battery courtesy of Berkeley Lab.

The gist of the problem is that sulfur tends to be soluble in the organic solvents used in conventional batteries. That process forms polysulfide ions — I know, right? — which can get to the lithium anode and re-solidify as precipitates, forming a barrier that interferes with storage capacity.

The result is that typical lithium-sulfur prototypes can’t last more than a dozen or so charge-recharge cycles without losing it, “it” being their ability to store a charge.

The Berkeley solution was to develop a nanomaterial composed of small particles of graphene flakes coated with sulfur, namely S-GO. As described by Berkeley writer Allan Chen, S-GO is characterized by a large, cavity-speckled surface area, which allows for more “intimate electronic contact” with sulfur while minimizing loss of contact with the current collector of the electrode.

When used as a cathode material in a lithium-sulfur battery, S-GO binds with lithium during discharge and releases it back to the anode during recharge.

Meanwhile, S-GO resolves some other key issues, including the massive bloating that bedevils lithium-sulfur technology. The graphene lends an element of flexibility that enables S-GO to accommodate the volume increase of up to 76 percent that sulfur suffers through as it is converted to lithium sulfide during discharge.

Electrochemical Teamwork Better EV Battery Range

Aside from the vastly improved cathode performance, the new battery sports such goodies as an enhanced binder (elastomeric styrene butadiene rubber combined with a thickener) that increases power density.

To deal with the polysulfide issue, the team used a coating of cetyltrimethyl ammonium bromide (a surfactant commonly used in drug delivery systems) on the sulfur electrode.

Also helping out with the polysulfides thing was a new electrolyte based on an ionic liquid, developed in-house at Berkeley (ionic liquids are non-volatile and non-flammable btw).

The new ionic liquid also provides a huge boost in the rate of battery operation, while increasing the speed of charging and the delivery of power during discharge.

Here’s the result as reported by Chen:

The battery initially showed an estimated cell-specific energy of more than 500 Wh/kg and it maintained it at >300 Wh/kg after 1,000 cycles—much higher than that of currently available lithium-ion cells, which currently average about 200 Wh/kg.

That puts the new battery’s potential well within sight of a 300-mile EV battery range:

For electric vehicles to have a 300-mile range, the battery should provide a cell-level specific energy of 350 to 400 Watt-hours/kilogram (Wh/kg). This would require almost double the specific energy (about 200 Wh/kg) of current lithium-ion batteries. The batteries would also need to have at least 1,000, and preferably 1,500 charge-discharge cycles without showing a noticeable power or energy storage capacity loss.

The next steps include increasing the use of sulfur, maintaining performance in extreme conditions, and of course, scaling up to size.

If there are any private sector partners out there to pitch in with the funding, Berkeley would love to hear from you so give them a holler. Remember, graphene is the miracle material of the new millenium.

Meanwhile, considering that Berkeley Lab is a Department of Energy facility, yes we built this!

About the Author

Tina Casey specializes in military and corporate sustainability, advanced technology, emerging materials, biofuels, and water and wastewater issues. Tina’s articles are reposted frequently on Reuters, Scientific American, and many other sites. Views expressed are her own. Follow her on Twitter @TinaMCasey and Google+.

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